Wires, such as typically used in the windings of transformers and motors, have been insulated with enamels applied from an organic solvent, the application of which can involve health, fire and air pollution hazards. To avoid such problems, the wire could instead be insulated by, for example, kraft paper, but such a process is typically slow and expensive, undesirably increases the bulk of the wire winding, and would be inapplicable in high temperature use areas, e.g., at 180.degree. C. and higher.
Of the high temperature enamels, the most common are based on polyesters, and especially modified polyesters such as polyester amides and polyester imides. While such materials have proven to have great utility because of their effective electrical insulating properties, physical toughness and resistance to heat and solvent, together with reasonable costs, the aforementioned disadvantages remain as drawbacks to the use of such enamels.
Accordingly, it has been deemed desirable to utilize a solvent-free process for the application of a film-type insulation to electrical conductors. One leading non-solvent process for such applications utilizes powder coatings. Currently, known powder systems are typically based on epoxy or polyester resins, and as such are capable of providing an insulation system which is equivalent to many enamels. For example, one such epoxy-based powder which has found excellent commercial acceptance is that disclosed in U.S. Pat. No. 4,267,300. Similarly, another epoxy-based composition is disclosed in U.S. Pat. No. 4,486,558. Such compositions are, however, limited to thermal classes of below about 130.degree. C. because of their base polymer composition. At temperatures above about 130.degree. C., coatings from these compositions can form cracks therein, resulting in dielectric failure. While a higher thermal class powder wire coating is highly desirable, I am unaware of the commercial availability of same.
In contrast with the foregoing, I have now discovered a composition which combines the high temperature characteristics of the 5-membered imide ring and the isocyanurate ring structures with the advantages and economies of a solventless powder coating system.
The composition is based on the combination of a solid carboxyl-terminated polyester resin produced from aromatic diacids and aliphatic glycols; a diimide diacid; and a triglycidyl isocyanurate. In this regard, it should be pointed out that while my composition need not contain conventional epoxy resins, the relatively fast epoxide/carboxyl reaction compares favorably with the fast cure properties known for epoxy resin/anhydride powder systems.
The use of the foregoing diimide diacid as a polyester component for enamels is disclosed in U.S. Pat. Nos. 4,145,334; 4,145,351; and 4,362,861. Furthermore, this diimide diacid has been disclosed as a polyester component in hot melt adhesive systems, namely in U.S. Pat. Nos. 4,038,254 and 4,075,179. In addition, the diimide diacid has been disclosed as a component in polyester imides used in preparing powders for coating wire in U.S. Pat. Nos. 4,117,032; 3,853,817; and 4,233,435.
In this prior art, the diimide diacid is taught to be copolymerized into a polyester, rather than used as a separate chemical constituent. Separate use allows for greater formulation flexibility. In addition, the elimination of the imide linkages in the polyester backbone enhance the flow properties of the resultant powder, since such linkages increase the melt viscosity of the polyester. Furthermore, the elimination of such linkages allows for the use of a higher molecular weight polyester than before available, resulting in enhanced flexibility of the ultimate coating.